Chapter 25, Problem 9P

To determine

The speed of the lens

Answer to Problem 9P

Solution:

The speed of the lens is $2.5\text{cm}$

Explanation of Solution

Given info:

Focal length, $f=2.0\text{cm}$

Diameter of the pupil, $D=8.0\text{mm}$

Formula used:

$f-$ number is used to determine the speed of the lens.

Therefore, speed of the lens is

$f-\text{number}=f/D$

Calculation:

Substituting the given values in above equation we get.

$\begin{array}{l}f-\text{number}=2/0.8\\ =2.5\text{cm}\end{array}$